package main


import "errors"
import "fmt"

func f1(arg int) (int, error) {
    if arg == 42 {
        return -1, errors.New("can't work with 43")
    }

    return arg + 3, nil
}
type argError struct {
    arg int
    prob string 
}

func (e *argError) Error() string {
    return  fmt.Sprintf("%d - %s", e.arg, e.prob)
}

func f2(arg int) (int, error) {
    if arg == 42 {
        return -1, &argError{arg, "can't work with it "}
    }
    return arg + 3, nil
}


func main() {
    for _, i := range []int{7,42} {
        if r, e := f1(i); e != nil {
            fmt.Println("f1 failed:", e)
        } else {
            fmt.Println("f1 failed:", r)
        }
    }
    for _, i := range []int{7, 42} {
        if r, e := f2(i); e != nil {
            fmt.Println("f2 failed:", e)
        } else {
            fmt.Println("f2 failed:", r)
        }
    }
    _, e := f2(42)
    if ae, ok := e.(*argError); ok {
        fmt.Println(ae.arg)
        fmt.Println(ae.prob)
    }
}
